The present invention relates to an integrated magnetic and electronic ballast for metal halide lamps. More particularly, the invention relates to a magnetic/electronic ballast designed to prolong the life of a metal halide lamp.
Existing illumination systems for high intensity discharge (HID) lamps, such as metal halide lamps, include conventional systems involving a core and coil arrangement and electronic ballast systems. The conventional systems are provided as an autotransformer and choke arrangement with an igniter and capacitor for power factor correction. The electronic ballasts are typically manufactured by Phillips Corporation, of Holland, and by Aromat Ltd. of Japan.
Metal halide lamps are very expensive and it would be desirable to prolong the lamp life as much as possible. As the line voltage changes or the lamp ages and its impedance characteristics change, therefore the lamp current, voltage and power change too. When the line voltage is increased, the lamp power is high. However, while the lamp provides greater brightness according to the Color Rendering Index (CRI), the result is a shorter lamp life, lamp and/or fixture damage, low color temperature, reduced efficiency, and electrode damage due to high curent. If the line voltage decreases, the lamp power is low, resulting in high color temperature, reduced CRI, low light output, which may damage the lamp, reducing its efficiency, since at low current the lamp fails to warm up. Thus, it is necessary to maintain the power constant.
The existing conventional type electromagnetic illumination systems, although they are very reliable, do not have any control of the current or voltage supply to the metal halide lamp, except for the so-called Constant Wattage Autotransformers (CWA) which provide regulation of about xc2x112% in relation to xc2x110% variations of the line voltage, but this is not good enough, and there is no control when the lamp ages.
The existing electronic ballasts do have control of the current and check the changes in current and voltage. In order to compensate for a change in current and lamp voltage, the illumination system maintains the power level constant by a smooth compensation function. However, these electronic ballasts are both more expensive and less reliable, since they have many electronic components that can malfunction. Additionally, the electronic ballasts use MOSFET switching techniques which operate at a high frequency and therefore generate radio frequency interference (RFI) effects. RFI effects are detrimental to the environment and have detrimental effects on other equipment. The electronic ballasts generally do not meet the standards that require a low level of RFI generation for any electrical device.
It would be desirable to provide a ballast for a metal halide lamp that would be economical, reliable, easy to manufacture and would increase lamp life while not generating RFI effects.
Accordingly, it is a broad object of the present invention to overcome the problems of the prior art and provide a metal halide ballast with improved properties.
In accordance with a preferred embodiment of the present invention there is provided a magnetic/electronic metal halide ballast with a compensation function for adjustment in autotransformer output voltage, which applies the compensation in various steps of voltage, to maintain the power constant. The metal halide ballast of the present invention provides constant monitoring of the autotransformer input voltage, enabling the level to be stepped by triac switching. The triac switching is controlled by a switching control provided by a microcontroller. Less components are required, making the present invention simpler, less expensive to manufacture and more reliable.
Other features and advantages of the invention will become apparent from the following drawings and the description.